Support protocol inference for Type[T] via metaclass (#14554)

Fixes #12553

This looks quite niche, but also it was mentioned recently couple times
for a real-life use case: enum classes, and implementation looks simple.

Author: ilevkivskyi

mypy/constraints.py

@@ -619,6 +619,17 @@ def visit_instance(self, template: Instance) -> list[Constraint]:
                         actual.item, template, subtype, template, class_obj=True
                     )
                 )
+            if self.direction == SUPERTYPE_OF:
+                # Infer constraints for Type[T] via metaclass of T when it makes sense.
+                a_item = actual.item
+                if isinstance(a_item, TypeVarType):
+                    a_item = get_proper_type(a_item.upper_bound)
+                if isinstance(a_item, Instance) and a_item.type.metaclass_type:
+                    res.extend(
+                        self.infer_constraints_from_protocol_members(
+                            a_item.type.metaclass_type, template, actual, template
+                        )
+                    )
 
         if isinstance(actual, Overloaded) and actual.fallback is not None:
             actual = actual.fallback

test-data/unit/check-protocols.test

@@ -3977,3 +3977,24 @@ class C:
     DEFAULT: ClassVar[C]
 
 x: P = C()
+
+[case testInferenceViaTypeTypeMetaclass]
+from typing import Iterator, Iterable, TypeVar, Type
+
+M = TypeVar("M")
+
+class Meta(type):
+    def __iter__(self: Type[M]) -> Iterator[M]: ...
+class Foo(metaclass=Meta): ...
+
+T = TypeVar("T")
+def test(x: Iterable[T]) -> T: ...
+
+reveal_type(test(Foo))  # N: Revealed type is "__main__.Foo"
+t_foo: Type[Foo]
+reveal_type(test(t_foo))  # N: Revealed type is "__main__.Foo"
+
+TF = TypeVar("TF", bound=Foo)
+def outer(cls: Type[TF]) -> TF:
+    reveal_type(test(cls))  # N: Revealed type is "TF`-1"
+    return cls()

test-data/unit/pythoneval.test

@@ -1858,3 +1858,22 @@ _testTupleWithDifferentArgsPy310.py:20: note: Revealed type is "builtins.list[Tu
 _testTupleWithDifferentArgsPy310.py:26: error: Invalid type: try using Literal[1] instead?
 _testTupleWithDifferentArgsPy310.py:27: error: Unexpected "..."
 _testTupleWithDifferentArgsPy310.py:30: note: Revealed type is "builtins.tuple[builtins.object, ...]"
+
+[case testEnumIterMetaInference]
+import socket
+from enum import Enum
+from typing import Iterable, Iterator, Type, TypeVar
+
+_E = TypeVar("_E", bound=Enum)
+
+def enum_iter(cls: Type[_E]) -> Iterable[_E]:
+    reveal_type(iter(cls))
+    reveal_type(next(iter(cls)))
+    return iter(cls)
+
+for value in enum_iter(socket.SocketKind):
+    reveal_type(value)
+[out]
+_testEnumIterMetaInference.py:8: note: Revealed type is "typing.Iterator[_E`-1]"
+_testEnumIterMetaInference.py:9: note: Revealed type is "_E`-1"
+_testEnumIterMetaInference.py:13: note: Revealed type is "socket.SocketKind"